1. The Field of the Invention
The present invention generally relates to optical transceiver modules and other optoelectronic devices. More particularly, the present invention relates to an internal shield for use in reducing electromagnetic interference emitted by such modules and devices by attenuating electromagnetic fields produced by components located therein.
2. The Related Technology
Fiber optics are increasingly used for transmitting voice and data signals. As a transmission medium, light provides a number of advantages over traditional electrical communication techniques. For example, light signals allow for extremely high transmission rates and very high bandwidth capabilities. Also, light signals are resistant to electro-magnetic interferences that would otherwise interfere with electrical signals. Light also provides a more secure signal because it doesn't allow portions of the signal to escape from the fiber optic cable as can occur with electrical signals in wire-based systems. Light also can be conducted over greater distances without the signal loss typically associated with electrical signals on copper wire.
While optical communications provide a number of advantages, the use of light as a transmission medium presents a number of implementation challenges. In particular, the data carried by a light signal must be converted to an electrical format when received by a device, such as a network switch. Conversely, when data is transmitted to the optical network, it must be converted from an electronic signal to a light signal. A number of protocols define the conversion of electrical signals to optical signals and transmission of those optical, including the ANSI Fibre Channel (FC) protocol. The FC protocol is typically implemented using a transceiver module at both ends of a fiber optic cable. Each transceiver module typically contains a laser transmitter circuit capable of converting electrical signals to optical signals, and an optical receiver capable of converting received optical signals back into electrical signals.
Typically, a transceiver module is electrically interfaced with a host device—such as a host computer, switching hub, network router, switch box, computer I/O and the like—via a compatible connection port. Moreover, in some applications it is desirable to miniaturize the physical size of the transceiver module to increase the port density, and therefore accommodate a higher number of network connections within a given physical space. In addition, in many applications, it is desirable for the module to be hot-pluggable, which permits the module to be inserted and removed from the host system without removing electrical power.
To accomplish many of these objectives, international and industry standards have been adopted that define the physical size and shape of optical transceiver modules to insure compatibility between different manufacturers. For example, in 2000, a group of optical manufacturers developed a set of standards for optical transceiver modules called the Small Form-factor Pluggable (“SFP”) Transceiver Multi-Source Agreement (“MSA”), incorporated herein by reference. In addition to the details of the electrical interface, this standard defines the physical size and shape for the SFP transceiver modules, and the corresponding host port, so as to insure interoperability between different manufacturers' products. There have been several subsequent standards, and proposals for new standards, including the XFP MSA for 10 Gigabit per second modules using a serial electrical interface, that also define the form factors and connection standards for pluggable optoelectronic modules, such as the published draft version 0.92 (XFP MSA), incorporated herein by reference.
As optical transmission speed provided by optoelectronic modules increases, additional problems arise. For example, electronic devices and components operating at high frequencies typically produce and emit electromagnetic fields that cause electromagnetic interference. This electromagnetic interference, referred to as _“EMI,”_ is undesired electrical noise resulting from the electromagnetic fields. The phenomenon is undesirable because EMI can interfere with the proper operation of other electrical components. Optical transceiver packages, especially those operating at high transmission speeds, typically include several such electronic devices and components and are therefore especially susceptible to emitting EMI. In particular, the physical configuration of existing transceiver modules does a poor job of containing EMI—especially as the generating speed of the module increases.
One attempt to limit EMI emissions from optoelectronic modules, such as optical transceiver modules, involves the use of EMI cages. EMI cages can be sized to fit about the exterior portion of the transceiver module and configured to intercept EMI that is emitted from electronic components contained within the transceiver module. Such cages, while effective at reducing EMI, are nonetheless accompanied by certain disadvantages. Among these is the additional design complexity of the transceiver operating environment that results from the use of such cages, as well as the additional space required for the cages in the operating environment. Additionally, the cage is adjacent the exterior of the transceiver module, which places it further away from the EMI sources that are located within the transceiver module. As EMI commonly follows a diverging path as it radiates away from its source, this requires relatively more cage material to be used to prevent EMI emission than if the EMI were attenuated closer to the EMI source.
In light of the above, therefore, a need exists in the art for a means by which EMI can be effectively reduced in optoelectronic modules, such as optical transceiver modules. Such means should be easily implemented while avoiding design complications of the transceiver operating environment. Further, any solution should avoid the use of excessive amounts of shielding material. In addition, it would be helpful to implement the solution in a manner that meets existing transceiver form factors.